Although seed packets suggest sowing in March, I would wait until at least the middle of April as beetroot need a bit of warmth to germinate. The wettest winter for forty years and I reckon your beetroot seeds have rotted off. You could sow early if under a cloche. Perhaps leave those already planted if you have the space and plant some more a little later.

Depends on whether you want big roots or small ones. Beetroot seed is a clustered group so you almost always get more than one germinating in the same place. Leave them be if you want smaller ones or thin them out if you want big ones.
I would not remove all three and replant, that gives them a check which can lead to them going straight to flower..bolting in other words.
We rarely even thin them out.

Sorry, I missed the point. As OB says they are multiple seeds so best wait and either let the three grow or thin out. I do believe that you can now buy seed which comprise of just a single one.

Thanks again, will remove the smaller seedlings. I was referring to the ones I'd set in my greenhouse which are now all growing, but I'll be setting more in my plot very soon.

Yes, betroot seeds are actually seed clusters, so you'll get more than one seedling popping up. You need to decide which one to keep and snip off the others with a pair of scissors to ensure that you get one strong plant rather than lots of weaker ones.

Oh and if you like Beetroot then can I suggest you try the different coloured ones? We like the white one very much.

Is there any difference in the taste ? White ones sound worth trying.

I find ordinary red beetroot to be bitter tasting (probably down to the tablets I have to take, they have the same effect on other vegetables too, especially Asparagus) but the white beets are much much sweeter. The yellow ones are sweeter than the red, but not as sweet as the white.
Never had them pickled as I prefer beets as they come rather than tasting of vinegar.

Thanks again, i will experiment with my beetroot this year
Mono, white, yellow etc.